Earth-boring tools for forming wellbores in subterranean earth formations generally include a plurality of cutting elements secured to a body. For example, fixed-cutter earth-boring rotary drill bits (also referred to as “drag bits”) include a plurality of cutting elements that are fixedly attached to a bit body of the drill bit. Similarly, roller cone earth-boring rotary drill bits may include cones that are mounted on bearing pins extending from legs of a bit body such that each cone is capable of rotating about the bearing pin on which it is mounted. A plurality of cutting elements may be mounted to each cone of the drill bit.
The cutting elements used in such earth-boring tools often include polycrystalline diamond compact (often referred to as “PDC”) cutting elements, which are cutting elements that include cutting faces of a polycrystalline diamond material. Polycrystalline diamond material is material that includes inter-bonded grains or crystals of diamond material. In other words, polycrystalline diamond material includes direct, inter-granular bonds between the grains or crystals of diamond material. The terms “grain” and “crystal” are used synonymously and interchangeably herein.
Polycrystalline diamond compact cutting elements are formed by sintering and bonding together relatively small diamond grains under conditions of high temperature and high pressure in the presence of a metal solvent catalyst (such as, for example, cobalt, iron, nickel, or alloys and mixtures thereof) to form a layer or “table” of polycrystalline diamond material on a cutting element substrate. These processes are often referred to as “high temperature/high pressure” (or “HTHP”) processes. The cutting element substrate may comprise a cement material (i.e., a ceramic-metal composite material) such as, for example, cobalt-cemented tungsten carbide. In such instances, the cobalt (or other catalyst material) in the cutting element substrate may be swept into the diamond grains during sintering and serve as a catalyst material for forming the inter-granular diamond-to-diamond bonds between, and the resulting diamond table from, the diamond grains. In other methods, powdered catalyst material may be mixed with the diamond grains prior to sintering the grains together in an HTHP process.
Upon formation of a diamond table using an HTHP process, catalyst material may remain in interstitial spaces between the grains of diamond in the resulting polycrystalline diamond table.
Differences in the thermal expansion between the diamond table and the cutting element substrate to which it is bonded may result in relatively large compressive and/or tensile stresses at an interface between the diamond table and the substrate that eventually lead to deterioration of the diamond table, cause the diamond table to delaminate from the substrate, or result in general ineffectiveness of the cutting element.
There are several conventional ways of making a PDC cutting element. For example, as shown in FIG. 1, a single HTHP cycle (e.g., in a diamond press 25) may be used to press diamond feed 10 (e.g., polycrystalline diamond grit bound together in a matrix) and a carbide substrate 20 together. The HTHP cycle simultaneously binds the material of the PDC feed 10 together to form a PDC table 30 and binds the PDC table 30 to the carbide substrate 20 to form a PDC cutting element 40. In another example, shown in FIG. 2, diamond feed 10 may be pressed in an HTHP cycle (e.g., in a diamond press 25) to form a preformed PDC table 50. The preformed PDC table 50 may then be attached to a carbide substrate 20 in another HTHP cycle (e.g., in a diamond press 25) to form a PDC cutting element 60. In either of the first two examples (FIGS. 1 and 2), the PDC table 30, 50 may include a cobalt (or other metal) catalyst and the carbide substrate 20 may include cobalt (or other metal) in a matrix phase. Thus, the PDC table 30, 50 and the carbide substrate 20 may be bound together by cobalt (or other metal) at the interface between the PDC table 30, 50 and the carbide substrate. In yet another example, shown in FIG. 3, a preformed PDC table 50 may be formed by pressing diamond feed 10 in an HTHP cycle (e.g., in a diamond press 25). The preformed PDC table 50 may be attached to a carbide substrate 20 by high temperature brazing to form a brazed PDC cutting element 70. The PDC table 50 and the carbide substrate 20 may be bound together by a braze alloy.
Conventional methods of attaching a PDC to a substrate, such as those described with reference to FIGS. 1-3, may have certain disadvantages. For example, stress may be induced at an interface between the PDC and the substrate during the pressing cycle. Stress may also be induced at the interface due to the high temperature of a brazing process. The level of stress at the interface that is ideal for stress propagation when the PDC cutting element is used (e.g., in boring the earth) may not be ideal for reducing the stress state after the pressing or brazing cycles.
Conventionally, PDC cutting elements including a PDC table and a carbide substrate are formed as described and then attached to the surface of a bit body of an earth-boring tool. The PDC cutting elements may be attached to the bit body using a brazing process. The brazing process can cause thermal stress or degradation of materials at the interface between the PDC cutting elements and the bit body.